The invention is in the field of semiconductor device manufacture, and relates more specifically to an improved technique for forming an oxide layer on a polycrystalline silicon (polysilicon) or monocrystalline silicon surface.
In the fabrication of both MOS and bipolar semiconductor devices, it is common to employ polysilicon or monocrystalline silicon layers for various purposes. Typically, a silicon dioxide (oxide) insulating layer is then grown over the silicon. This basic technique is used to fabricate numerous types of devices, including MOS and bipolar transistors, memory cells and capacitors.
However, it is well known in this art that there are certain problems associated with thermal oxide growth on silicon surfaces. For example, when oxide layers are grown on polysilicon surfaces at relatively high temperatures, above about 1000.degree. C., the resulting dopant outdiffusion causes problems with device fabrication. These problems will only become greater as geometries shrink and packing densities increase in future generations of devices. On the other hand, when oxide layers are thermally grown on polysilicon surfaces at relatively low temperatures, below about 1000.degree. C., various defects, including asperities in the polysilicon-oxide interface, silicon inclusions in the oxide layer, and cusps at the corners of the polysilicon, may be present. Thus, reducing the oxidation temperature in order to minimize dopant outdiffusion problems will result in various irregularities at and near the polysilicon-oxide interface, which will in turn reduce the dielectric breakdown voltage and reduce device reliability and yield. It has been determined that these irregularities are caused by stresses in the oxide layer, and that these stresses, and the resulting irregularities, increase with decreasing oxidation temperature.
A more detailed discussion of the interface problems associated with growing oxide on a polysilicon surface is contained in "Polysilicon/SiO.sub.2 Interface Microtexture and Dielectric Breakdown", R. B. Marcus, T. T. Sheng, and P. Lin, J. Electrochem. Soc.: SOLID-STATE SCIENCE AND TECHNOLOGY, Vol. 129, No. 6, pp. 1282-1289, June, 1982. Additionally, problems associated with low-temperature oxidation of nonplanar silicon and polysilicon surfaces are discussed in "The Oxidation of Shaped Silicon Surfaces", R. B. Marcus and T. T. Sheng, J. Electrochem. Soc.: SOLID-STATE SCIENCE AND TECHNOLOGY, Vol. 129, No. 6, pp. 1278-1282, June, 1982. These references clearly describe the problems associated with low-temperature oxide growth, but offer no solution other than the obvious one of increasing the oxidation temperature. However, as noted above, this solution is not without its problems, as high-temperature oxide growth results in increased dopant outdiffusion, which is a significant problem today and which will become even more of a problem as device geometries shrink and packing density increases in the future.
It would therefore be desirable to have a process which avoids dopant outdiffusion problems associated with high oxidation temperatures, and at the same time avoids the stress-induced irregularities associated with low oxidation temperatures which degrade breakdown voltage, yield and reliability.